Injection molded solder bumping

ABSTRACT

Methods for depositing material on a chip include forming a mold layer. The mold layer includes one or more openings over respective contact areas, each of the one or more openings having an upper volume and a lower volume. The upper volume has a smaller diameter than a diameter of the lower volume. Each contact area is within the respective lower volume. A material is injected into the one or more openings under pressure.

BACKGROUND Technical Field

The present invention generally relates to injection molded solderingand, in particular, to the use of mask cavities to reduce the pressureneeded to inject solder.

Description of the Related Art

Injection molded soldering injects molten solder into vacuumed cavitieson a patterned resist mask. Referring now to FIG. 1, a cross-sectionalview of a conventional injection molded soldering process is shown. Acontact pad 108 represents the under-bump metal (UBM) on a substrate102. A resist mask 104 is formed around the contact pad 108, with a gapover the contact pad 108. The injection head 106 passes over the resistmask 104, injecting solder 110 into the cavity.

However, residual gas may remain in the cavity and have a negativeeffect on how the solder 110 fills the cavity. For example, bubbles mayremain in the solder that weaken the solder joint and/or impedeconductivity. When the molten solder makes contact with the UBM 108,residual gas can be removed by wetting of the solder on the UBM 108. Ifno such contact is made, then the residual cannot be removed by suchwetting. It is difficult to remove this gas entirely, because there isoften leakage between the solder injection head 106 and the resist mask104.

One way to improve solder contact is to increase the injection pressure.However, higher injection pressure necessitates a correspondingly highinjection head pressure, which can negatively affect the solderformation by deforming the mask 104. This puts an upper limit on howmuch pressure can realistically be applied by the injection head. Forfiner pitch bumping, even higher injection pressures are needed.

SUMMARY

A method for depositing material on a chip includes forming a moldlayer. The mold layer includes one or more openings over respectivecontact areas, each of the one or more openings having an upper volumeand a lower volume. The upper volume has a smaller diameter than adiameter of the lower volume. Each contact area is within the respectivelower volume. A material is injected into the one or more openings underpressure.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description will provide details of preferred embodimentswith reference to the following figures wherein:

FIG. 1 is a cross-sectional diagram of prior art solder injection;

FIG. 2 is a cross-sectional diagram of low-pressure solder injection inaccordance with the present embodiments;

FIG. 3 is a cross-sectional diagram of a step in the formation of maskcavities for low-pressure solder injection in accordance with thepresent embodiments;

FIG. 4 is a cross-sectional diagram of a step in the formation of maskcavities for low-pressure solder injection in accordance with thepresent embodiments;

FIG. 5 is a cross-sectional diagram of a step in the formation of maskcavities for low-pressure solder injection in accordance with thepresent embodiments;

FIG. 6 is a cross-sectional diagram of a step in the formation of maskcavities for low-pressure solder injection in accordance with thepresent embodiments;

FIG. 7 is a cross-sectional diagram of a step in the formation of maskcavities for low-pressure solder injection in accordance with thepresent embodiments;

FIG. 8 is a cross-sectional diagram of a step in the formation of maskcavities for low-pressure solder injection in accordance with thepresent embodiments;

FIG. 9 is a cross-sectional diagram of a step in the formation of maskcavities for low-pressure solder injection in accordance with thepresent embodiments;

FIG. 10 is a cross-sectional diagram of a cavity for low-pressure solderinjection in accordance with the present embodiments;

FIG. 11 is a cross-sectional diagram of a cavity for low-pressure solderinjection in accordance with the present embodiments; and

FIG. 12 is a block/flow diagram of a method for low-pressure solderinjection in accordance with the present embodiments.

DETAILED DESCRIPTION

Embodiments of the present invention provide injection molded solderapplication that uses resist masks with cavities which provide gasdisplacement during injection. These cavities thereby decrease theamount of pressure needed to inject the solder material, which in turndecreases the amount of injection head pressure that needs to be appliedand decreases the potential for damage and deformity in the resist mask.

In a first embodiment, the cavities are formed in the resist mask bycreating the resist mask in a two-layer process. In another embodiment,the cavities are formed by defocusing a beam of curing light duringexposure, such that the width of the cavity changes. In addition,multiple embodiments for the cavity structures are provided herein.

Referring again to FIG. 1, once the injection head 106 in a conventionalinjection molded solder system covers the cavity and solder begins to beinjected, the gas in the cavity can be sealed in the cavity. At thispoint, any gas remaining in the cavity will be compressed as additionalsolder 110 is injected and occupies additional volume in the cavity. Atsome point, the pressure of the compressing gas may equal the injectionpressure, which is limited by the amount of downward force applied bythe injection head 106. If insufficient injection pressure is applied,the solder 110 may never contact the contact pad 108, leaving an air gapin the cavity that prevents an electrical connection from being formed.

To avoid this problem, the present embodiments increase the volume ofthe cavity at its base. This gives space for the gas to displace into asthe solder 110 fills the cavity, thereby lowering the pressure of thegas and decreasing the amount of downward pressure that the injectionhead 106 needs to exert. The amount of pressure that is needed for thesolder 110 to make contact with the contact pad 108 will depend on thedimensions of the cavity.

Referring now to FIG. 2, a cross-sectional view of an injection moldedsoldering process is shown. In particular, this embodiment shows anextended cavity 202 which has a width in a bottom portion that isgreater than that of the contact pad 108 and a width in an upper portionthat is the same as that of the contact pad 108. It should be understoodthat the width of the upper portion may alternatively be greater or lessthan that of the contact pad 108.

In this embodiment, as the solder 110 pushes down into the cavity 202,the remaining gas inside the cavity 202 compresses and the pressureinside the cavity 202 rises. However, because the cavity 202 has agreater volume than a conventional cavity would, the gas partiallydisplaces into the extended portions to the left and right of thecontact pad 108. As a result, the pressure that the gas reaches by thetime the solder 110 comes into contact with the contact pad 108 is belowthe limit of pressure that the injection head 106 can safely exert.

The dimensions of the cavity 202 may be selected to allow the solder 110to make contact with the contact pad 108 without over-filling to thepoint where the solder 110 pushes laterally past the boundaries of thecontact pad 108. The thicker the resist mask 104 is, the more gas willbe trapped and, therefore, the wider the cavity 202 will need to be toaccommodate the displaced gas at pressures that are suitable for solderinjection.

When determining the dimensions for the cavity 202, including thedimensions for the additional volume, parameters that influence thedesign include the expected initial pressure of the remaining gas, theexpected contact angle between the molten solder and the mold sidewall,and the resist thickness. With thicker resist layers, the vacant spacein the mold cavities can be made taller, thereby providing more volumefor displaced gas. More volume therefore means that the pressure neededto inject solder decreases. The present embodiments can provide anyarbitrary degree of decrease the injection pressure that is needed.

Referring now to FIG. 3, a cross-sectional view of a step in aninjection molded soldering process is shown. A semiconductor substrate302 is provided with a series of contact pads 108. It is specificallycontemplated that the semiconductor substrate 302 may be abulk-semiconductor substrate. In one example, the bulk-semiconductorsubstrate may be a silicon-containing material. Illustrative examples ofsilicon-containing materials suitable for the bulk-semiconductorsubstrate include, but are not limited to, silicon, silicon germanium,silicon germanium carbide, silicon carbide, polysilicon, epitaxialsilicon, amorphous silicon, and multi-layers thereof. Although siliconis the predominantly used semiconductor material in wafer fabrication,alternative semiconductor materials can be employed, such as, but notlimited to, germanium, gallium arsenide, gallium nitride, cadmiumtelluride and zinc selenide. Although not depicted in FIG. 3, thesemiconductor substrate 302 may also be a semiconductor on insulator(SOI) substrate. Non-semiconductor substrate materials may furthermorebe employed instead of a semiconductor material.

The contact pads 304 are formed from a conductive material. For example,while the contact pads 304 are often formed from copper, alternativematerials may include tungsten, nickel, titanium, molybdenum, tantalum,platinum, silver, gold, rubidium, iridium, rhenium, ruthenium, andalloys thereof. It is specifically contemplated that the contact pads304 provide an electrical interface to components formed on or in thesubstrate 302, but contact pads 304 may alternatively be included solelyfor structural contacts. The contact pads 304 may be formed on thesubstrate 302 by any appropriate process such as, e.g., electrolessplating. In one specific embodiment, electroless deposition of coppermay rely on the presence of a reducing agent, for example formaldehyde,which reacts with the copper metal ions to deposit the metal. In afurther embodiment, the contact pads 304 may be deposited using asputter process.

Referring now to FIG. 4, a cross-sectional view of a step in aninjection molded soldering process is shown. A first resist layer 402 isdeposited over the substrate 302 and the contact pads 304. It isspecifically contemplated that the first resist layer 402 may be formedfrom, e.g., polymethylglutarimide (PMGI) or any other appropriatelift-off resist material. The first resist layer 402 may be depositedusing a spin-on process to evenly distribute the resist material acrossthe substrate 302. The first resist layer 402 may then be soft-baked at,e.g., about 150° C. to about 200° C. to partially harden the firstresist layer 402.

Referring now to FIG. 5, a cross-sectional view of a step in aninjection molded soldering process is shown. A second resist layer 502is formed on top of the first resist layer 402. Any appropriate imagingresist material may be used. The second resist layer 502 is depositedusing, e.g., a spin-on process and is then soft-baked in place.

Referring now to FIG. 6, a cross-sectional view of a step in aninjection molded soldering process is shown. The second resist layer 502is patterned by, e.g., exposing the resist to ultraviolet light or byany other patterning process that is appropriate to the material of thesecond resist layer 502. The exposure chemically changes the exposedregions 602.

Referring now to FIG. 7, a cross-sectional view of a step in aninjection molded soldering process is shown. The exposed regions 602 ofthe second resist layer 502 and portions of the first resist layer 402underlying the exposed regions 602 are developed and etched away. Theremoval of this material leaves behind cavities 702 in the resist layers402 and 502 for subsequent solder injection.

It should be noted that the material of the first resist layer 402 neednot be separately exposed to a curing or patterning light. Instead, thematerial may be removed using, e.g., a timed, isotropic etch thatselectively removes material from the first resist layer 402 withoutharming the material of the second resist layer 502.

It should be noted that the above is just one embodiment for theformation of cavities. Instead of the above, any appropriate method forforming cavities to accommodate gas pressure may be used. Several moresuch embodiments are discussed herein, and it should be understood thatthe methods of forming the cavities are intended to be illustrative andnot limiting. Regardless of the particular shape of the cavities or themethod of fabrication of the mask, solder injection for all of thepresent embodiments is performed according to the same process.

Referring now to FIG. 8, a cross-sectional view of an alternative stepin an injection molded soldering process is shown. In this embodiment,the first resist layer 802 is soft-baked and exposed before the secondresist layer 804 is deposited, creating exposed regions 806. The secondresist layer 804 is then coated over the surface of the first resistlayer 802. Some candidates for the materials of the first resist layer802 and the second resist layer 806 include, e.g., novolak resin andphenol resin, though it should be understood that alternative materialsmay be used instead.

Referring now to FIG. 9, a cross-sectional view of an alternative stepin an injection molded soldering process is shown. After the firstresist layer 802 is exposed and the second resist layer 804 is formed,the second resist layer 804 is exposed. The exposed regions 902 of thesecond resist layer 804 are directly above the exposed regions 806 ofthe first resist layer 802. The mask used to define the exposed regions902 of the second region 804 will also block regions outside the exposedregions 902 over the first resist layer 802. The exposed regions 806 and902 of the first resist layer 802 and the second resist layer 804respectively are then etched away, leaving cavities such as those shownin FIG. 7.

Referring now to FIG. 10, a cross-sectional view of an alternative stepin an injection molded soldering process is shown. In this embodiment, asingle resist layer 1002 is deposited. During exposure, the light beamis defocused, resulting in the beam spreading as it nears the substrate302. The shape of the exposed region 1004 is wider near the substrate,such that when the exposed material is removed, cavities will remain toaccommodate increased gas pressure.

Referring now to FIG. 11, a cross-sectional view of an alternative stepin an injection molded soldering process is shown. In this embodiment, afirst resist layer 1102 and a second resist layer 1104 are patterned byany appropriate process to create a cavity 1106 that has a width in thefirst resist layer 1102 that is about the same width as the contact pad.In this example, the height of the lower portion of the cavity 1106 isgreater than the expected height for the injected solder. The additionalvertical height provides room for the compressed gas to displace into,thereby allowing the injected solder to make contact with the contactpad.

It is to be understood that aspects of the present invention will bedescribed in terms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps can be varied within the scope of aspects of the presentinvention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements can also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements can be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments can include a design for an integrated circuitchip, which can be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer can transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein can be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case, the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

It should also be understood that material compounds will be describedin terms of listed elements, e.g., SiGe. These compounds includedifferent proportions of the elements within the compound, e.g., SiGeincludes Si_(x)Ge_(1-x) where x is less than or equal to 1, etc. Inaddition, other elements can be included in the compound and stillfunction in accordance with the present principles. The compounds withadditional elements will be referred to herein as alloys.

Reference in the specification to “one embodiment” or “an embodiment”,as well as other variations thereof, means that a particular feature,structure, characteristic, and so forth described in connection with theembodiment is included in at least one embodiment. Thus, the appearancesof the phrase “in one embodiment” or “in an embodiment”, as well anyother variations, appearing in various places throughout thespecification are not necessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This can be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments. As used herein, the singular forms “a,” “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises,” “comprising,” “includes” and/or “including,” when usedherein, specify the presence of stated features, integers, steps,operations, elements and/or components, but do not preclude the presenceor addition of one or more other features, integers, steps, operations,elements, components and/or groups thereof.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” and the like, can be used herein for ease of description todescribe one element's or feature's relationship to another element(s)or feature(s) as it illustrated in the FIGS. will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the FIGS. For example, the device in the FIGS.is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the term “below” can encompass both an orientation ofabove and below. The device can be otherwise oriented (rotated 90degrees or at other orientations), and the spatially relativedescriptors used herein can be interpreted accordingly. In addition, itwill also be understood that when a layer is referred to as being“between” two layers, it can be the only layer between the two layers,or one or more intervening layers can also be present.

It will be understood that, although the terms first, second, etc. canbe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another element. Thus, a first element discussed belowcould be termed a second element without departing from the scope of thepresent concept.

Referring now to FIG. 12, a process for solder injection is shown. Block1202 forms the contact pads 108 on the substrate 102. Block 1204 forms aresist layer over the contact pads. It should be noted that the resistlayer may be a single-layer resist or a double-layer resist as describedin the various embodiments above. It should furthermore be noted thatother configurations of resist are also contemplated and may be employedin accordance with design needs. Block 1206 forms a solder mold in theresist layer over each contact pad 108. The solder molds each havemultiple diameters, with at least one diameter being the same as adiameter of the contact pads. In some embodiments, an upper portion ofthe mold has a greater diameter than a lower portion of the mold, insome embodiments a lower portion of the mold has a greater diameter thanan upper portion of the mold, and in still other embodiments thediameter of the mold changes continuously along the height of the mold.In all embodiments, the mold includes at least one cavity in addition tothe volume needed to accommodate the injected solder. The additionalvolume of the cavity accommodates gas that is displaced by the injectedsolder 110.

Block 1208 positions an injection head 106 on the resist layer. Pressureis applied by the injection head 106 on the resist layer to force theinjection head 106 to remain in contact with the resist layer whilesolder is injected under pressure in block 1210. The solder 110 pushesdown into the mold to make contact with the contact pad 108. In doingso, any gas remaining in the mold is compressed and displaced into theadditional volume of the cavities.

Block 1212 determines whether there are any unfilled solder molds. Ifso, block 1213 moves the injection head 106 to the next solder mold andblock 1210 injects solder into that solder mold. This process continuesuntil all of the solder molds are filled. Block 1214 then strips theresist layer. Block 1216 reflows the solder, forming solder balls thatare suitable for use in mounting other device components.

Having described preferred embodiments of a system and method (which areintended to be illustrative and not limiting), it is noted thatmodifications and variations can be made by persons skilled in the artin light of the above teachings. It is therefore to be understood thatchanges may be made in the particular embodiments disclosed which arewithin the scope of the invention as outlined by the appended claims.Having thus described aspects of the invention, with the details andparticularity required by the patent laws, what is claimed and desiredprotected by Letters Patent is set forth in the appended claims.

What is claimed is:
 1. A method for depositing material on a chip,comprising: forming a mold layer, the mold layer comprising one or moreopenings over respective contact areas, each of the one or more openingsincluding an upper volume and a lower volume, the upper volume having asmaller diameter than a diameter of the lower volume, wherein eachcontact area is within the respective lower volume; and injecting amaterial into the one or more openings under pressure.
 2. The method ofclaim 1, wherein forming the mold comprises depositing only a singleresist layer on the substrate.
 3. The method of claim 2, wherein formingthe mold layer further comprises exposing the resist layer over therespective contact areas, defocusing a light beam during exposure. 4.The method of claim 1, wherein forming the mold comprises depositing afirst resist layer over a substrate and a second resist layer over thefirst resist layer.
 5. The method of claim 4, wherein forming the moldfurther comprises exposing the second resist layer in a volume aboveeach respective contact area.
 6. The method of claim 5, wherein formingthe mold further comprises etching away the exposed volume of the secondresist layer and underlying material of the first resist layer to formthe one or more openings.
 7. The method of claim 5, wherein forming themold further comprises exposing the first resist layer before depositingthe second resist layer over the first resist layer.
 8. The method ofclaim 1, wherein the diameter of the upper volume is substantiallysimilar to a diameter of the contact areas.
 9. The method of claim 1,wherein the diameter of the lower volume is substantially similar to adiameter of the contact areas.
 10. The method of claim 1, wherein themolten material is solder and wherein injecting the molten solder causesthe molten solder to wet onto the contact areas when the molten soldermakes contact with the contact areas.